Rotational relaxation rates in CO-He mixtures

The rotational level populations of CO molecules were measured in CO(<10%) + He free jets by electron beam fluorescence (in a stationary jet) and resonantly enhanced multiphoton ionization (in a pulsed jet). The measured evolution of the non-equilibrium rotational energy was used to derive the rotational relaxation cross-sections in the temperature range from 6 K to 140 K. To compare and analyse on a common basis all available experimental data (ours and others) on rotational relaxation of CO in He, the infinite order sudden approximation was explored. The following quantities were investigated: integral rotationally inelastic cross-sections, state-to-state rate coefficients, rotational relaxation times, line broadening coefficients, and non-equilibrium rotational energy distributions in a free jet.